Conventional photoreceptors used in electrophotography may be grouped into two types of photoreceptors depending on whether inorganic or organic photoconductive material is used in the photoconductive layer. Organic photoconductive materials are widely used since they offer certain advantages such as high production rate and low manufacturing costs.
Recently, a so-called "function separated" type photoreceptor, in which the various functions and capabilities of the photoreceptor are divided and distributed among and between plural component layers of the photoreceptor system, has gained popularity. A variety of various systems have been proposed with a view towards improving the quality and characteristics of such photoreceptors including charge retention, stability to cyclic operation, sensitivity, spectral characteristics and mechanical strength.
A function separated organic photoreceptor comprises a charge generating layer and a charge transporting layer formed on an electrically conductive substrate. It is known that organic pigments such as bis-azo pigments and condensed polycyclic quinone pigments are suitable for use as charge generating materials in the charge generating layer. Organic pigments, however, generally have a low charge generating efficiency, and photoreceptors which utilize the above organic pigments in the charge generating layer are not completely satisfactory in terms of sensitivity.
In attempting to resolve these problems, several proposals have been made to replace the organic pigments with inorganic photoconductive materials such as selenium or selenium alloys as in, for example, JP-A-52-120834 and JP-A-53-27033 (The term "JP-A" as used herein means an "unexamined, published Japanese patent application"). Although these approaches are effective in achieving acceptable levels of photoreceptor sensitivity, there remains a need to improve the stability of cyclic operations.
Suitable charge transporting materials for use in the charge transporting layer include, for example, amine compounds, pyrazoline compounds, oxadiazole compounds, carbazole compounds and hydrazone compounds. However, it has not been possible to use these charge transporting materials in conjunction with the above-described charge generating materials to produce a function separated, layered photoreceptor which ensures satisfactory performance in practical electrophotographic applications.
There are two basic prerequisites to attaining satisfactory electrophotographic performance with conventional function separated, layered photoreceptors having a charge generating layer and a charge transporting layer. The first requirement is efficient charge generation in response to light absorption due to the charge generating materials. The second requirement is efficient injection and transport of the generated charge across the charge transporting material. If only the first requirement is satisfied, the photoreceptor cannot respond to light in a satisfactory manner.
In some instances, the photoreceptor has the charge transporting layer positioned on top of the charge generating layer with light exposure originating from the charge transporting layer side. In such a photoreceptor, it is important that the charge transporting layer be adequately transparent in order to produce a photoreceptor having a high level of sensitivity, i.e., light must pass through the charge transporting layer and reach the charge generating layer without substantial reflection and/or absorption by the charge transporting layer.
Although a variety of various charge generating and charge transporting materials have been used in fabricating electrophotographic photoreceptors, it nevertheless remains an extremely difficult proportion to select combinations of charge generating and charge transporting materials that satisfy not only the two basic prerequisites as mentioned above but also all other relevant criteria including electrophotographic characteristics, such as sensitivity, acceptance potential, charge retention, potential stability, residual potential and spectral characteristics; operating characteristics, such as strength, durability and smudge resistance; as well as consistency in the production and quality of applied coatings.
Currently, there are several known charge generating materials which satisfy the first prerequisite of efficient charge generating in response to light absorption. One of these is a bis-azo pigment such as chlorodian blue, described in JP-A-57-144557 which is known to exhibit a relatively high charge generation efficiency. Such pigments, however, do not exhibit sufficient photo-response and therefore are not suitable for use in high-speed copying machines.
As previously discussed, efficient charge generation by the charge generating layer is one of two prerequisites for satisfactory electrophotographic performance. That is, satisfactory electrophotographic performance cannot be achieved without also having charge transporting materials that can efficiently inject and transport the generated charges across the charge transporting layer. It is known that, in some cases, the injection of generated charges into the charge transporting layer bears a relationship and is therefore affected by the difference in ionization potential between the charge generating material and charge transporting material. This applies, however, only to a limited number of materials, and no clear relationship has yet been established which correlates this relationship with electrophotographic characteristics and performance. It can, however, be said that the efficiency of charge injection is largely dependent upon the characteristics of the charge generating and charge transporting materials used in combination. There are a variety of factors which change from one type of material to the other and which, therefore, have varying effects on charge injection depending on the particular charge generating and charge transporting materials used in making the photoreceptor.